Pulsed laser systems comprising an oscillator and high power fiber amplifier(s) are well known in the art. See, for example, U.S. Pat. Nos. 6,208,458; 6,181,463; and 5,696,782. Conventional systems typically operate at a fixed pulse width and repetition rate. However, systems with variable pulse widths and repetition rates have also been developed. For example, see U.S. Pat. Nos. 6,347,007; 6,335,941; and 6,433,306 to Grubb et al. For certain applications, it is desired that the output pulse energy of these types of systems be maintained at a constant value as the pulse width and repetition rate of the pulse source is varied. This problem is addressed in U.S. Pat. No. 7,505,196 which discloses methods and systems for controlling and protecting pulsed high power fiber amplifier systems.
The gain of the amplifier system depends on the rate at which energy is stored into, and depleted from, the doped fiber. Therefore, output pulse energy varies as a function of the rate of energy storage into the amplifier and the repetition rate of the seed, which extracts energy from the amplifier. A known method includes adjusting the gain of the amplifier by adjusting the power of the amplifier pump diode by changing its drive current as a function of the seed source pulse energy and repetition rate. A further alternative, which is suitable under some conditions, includes changing the pulse energy of the seed source by modulating the source directly or attenuating its input to the power amplifier.
Methods for controlling and protecting amplifiers from undesirable operation modes of modelocked lasers are described in U.S. Pat. No. 7,505,196, entitled: “Method and apparatus for controlling and protecting pulsed high power fiber amplifier systems”, assigned to the assignee of the present invention, and hereby incorporated by reference in its entirety.
U.S. Pat. No. 6,839,363, entitled “Digital control of actively mode-locked lasers” discloses a processor that digitally extracts noise information from multiple monitor signals generated from the laser output. Control functions are achieved with one or more bandpass filters covering various portions of the frequency range of the laser output. In U.S. Pat. No. 6,839,363, a microprocessor digitally diagnoses the operating condition of the laser based on the noise information. Mode locking may be maintained by reducing the noise. An array of A/D and D/A converters allow for digital control of the active mode-locked laser.
Takara et al, in “Stabilisation of a modelocked Er-doped fibre laser by suppressing the relaxation oscillation frequency component”, Electronic Letters, 16 Feb. 1995, Vol. 31 No. 4, 292-293, disclosed a stabilizing method based on the observation that the relaxation oscillation RF power of the detected output pulses is a good measure of instability. Stable bit-error-free operation at 6.3 GHz for long periods was reported. The bandwidth of the optical receiver was set in the range from 10 Hz to 200 kHz. The RF power ratio between the 6.3 GHz mode-locking frequency and background noise exceeded 70 dB.
Mode-locked laser systems are also widely used in frequency metrology, where the mode-locked laser acts as a frequency comb that can be used for precision measurements. Generally, such frequency combs are susceptible to noise. Low noise operation of the mode-locked laser can further improve precision. For example, high sensitivity detection and elimination of rapid multiple (double) pulsing while providing low latency for high speed execution of control functions is desirable.
A need exists for high sensitivity diagnostic tools for use with mode-locked lasers, particularly for application to frequency comb metrology. It is desirable to distinguish noisy and quiet operation of mode-locked lasers, and to rapidly detect pulse instabilities such as double pulses, period doubling, Q-switch mode-locking and other phenomena which degrade the quality of mode-locked signals, and which may occur over a wide frequency range.